Electrocardiograph (ECG) systems measure cardiac electrical activity associated with the muscular pumping activity of the heart. The electrical activity is measured by contacts or leads placed on the body of the patient. Typically, the measured electrical activity may then be printed out as an ECG waveform or trace for review by a doctor or diagnostician.
In hospitals, added functionality and workflow integration may be provided by digital ECG systems that acquire and store the ECG data in a digital format. The digital ECG data may be stored on various magnetic or optical devices, may be transmitted to one or more display stations remote from the patient, and may be printed once or numerous times from the stored record. The digital ECG data, therefore, provides a degree of flexibility, security, and reproducibility that may not be easily obtained from non-digital ECG system that produce only a paper record of the ECG waveform or trace.
ECG data in raw form may be too large for storage or transmission. Data compression may be applied to reduce the size of the ECG data and thus reduce the storage space requirement or transmission bandwidth requirement. When data compression is used, additional processing is required before storage or transmission to compress the data and additional processing is also required after retrieval to decompress or reconstruct the data. Data compression may be either lossless or lossy. With lossless data compression the decompressed data is identical to the original data. With lossy data compression the decompressed data may not be identical to the original data, but the goal is to make it as close as possible. When lossy compression is used the compression rate, or data size reduction, can be much larger. Whether lossless or lossy compression is appropriate depends highly on the particular application. For ECG signal compression lossless compression is generally more desirable, though lossy compression is sometimes acceptable.
Despite the benefits of digital ECG, certain vulnerabilities may be created for hospitals using the digital technology. For example, in a remote monitoring application, strict lossless reconstruction of a compressed ECG signal cannot be a guaranteed in conjunction with fixed rate transmission and real-time delivery. In particular, meeting these three criteria conflict in some circumstances. For example, it is not possible to absolutely guarantee lossless delivery of an arbitrary signal under the other two requirements (assuming, of course, that the fixed-rate available is less than what is required to send the signal samples in raw, uncoded, form). Any lossless compression system can fail under these constraints for certain ECG signals.
For example, if the ECG signal becomes very noisy over a short duration, then a lossless compressor will produce more compressed bits for that duration. If the fixed rate constraint is to be met, then the transmission delay must increase. If the transmission delay is to be met, then the fixed rate must be increased. When neither is possible, data will be lost.